EVEREST June, 2013 - California Department of Boating and ...

Orange County Coastal Regional Sediment Management Plan
excessive erosion. Construction of channels, roads, and buildings has been known to both
reduce and increase erosion. Hardening of the watershed reduces the amount of erodible area,
but increases stormflow runoff by limiting infiltration. These higher flows can lead to streambank
downcutting, eroding banks, and filling of silt basins. Overall, these mechanisms lead to
decreases in the amount of coarse sediment available for delivery to the beaches via streams.
Some coastal structures such as harbors, jetties, groins, and breakwaters alter movement of
sediment along the shoreline while other coastal structures such as riprap and seawalls can
reduce the amount of sediment supplied directly to the shoreline through the reduction of bluff
and cliff erosion. These process changes are shown within one schematic sedimentshed in
Figure 2.1.
Human modifications to the coastal watersheds and shorelines of California have resulted in the
following sediment-related problems, some of which are exacerbated by climatic and sea level
changes associated with global warming:
• Beaches are undergoing accelerated erosion, reducing recreational opportunities,
contributing to habitat loss, and increasing the probability of coastal storm damage;
• Coastal wetlands and lagoons are experiencing accelerated sedimentation and/or
erosion;
• Sediment is being removed, trapped, redirected, modified, and polluted as it moves
through coastal watersheds to the shoreline and along the coast;
• Sand dredged from harbor channels are, in many instances, placed at locations that do
not optimize the coastal beneficial use of the material; and
• Sediment supply to the coast has been, and continues to be, reduced as a result of
interruptions caused by dams and debris basins, sand and gravel mining, artificially
stabilized shoreline, and hardened coastal watersheds.
2.1.2 Solution – Coastal RSM Plans
Many watershed and shoreline problems caused by human modifications to the coast can be
solved and/or addressed through implementation of RSM. A schematic representation of RSM
as developed by the CSMW (Everest, 2007) is shown for one typical sedimentshed in
Figure 2.2. These strategies are:
• Sediment can be bypassed around upstream dams and blockages and be transported
either to immediately downstream of the dam or directly to the coast;
• Sand and gravel mining activities can be moved away from river beds to prevent sand
loss in river beds. For sand loss due to sand and gravel mining in the river, the loss can
be compensated through projects or mitigation fees;
• Sand held in place through urbanization can be compensated through projects or fees;
Everest International Consultants, Inc. 2.2